Re: Capacitive water level sensor

On Tue, 13 Jun 2006 20:24:53 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On 14 Jun 2006 03:02:11 GMT,
pbdelete@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx wrote:

I've often pondered using a piece of PVC pipe as if it were an organ
pipe, driving it with a speaker, finding its resonance, and
determining depth from that.

Then you would need to compensate for humidity/temperature I guess ..?

I doubt that humidity has much effect on resonance frequency... Q
maybe, resonance frequency, no. Likewise temperature over a body of
water should be relatively stable.

Considering the most basic form of the small-signal speed of sound in
an ideal gas equation:

C = sqrt[k*pressure/density]

Where C is the small-signal speed of sound, k is the ratio of specific
heats (specific heat at constant pressure divided by specific heat at
constant volume) and pressure and density are of the gas without
disturbance (subsitute ideal gas law relations for other forms).

Humidity has an effect on the small-signal speed of sound only in that
it changes the density of the air, the exact same reason temperature
changes the small-signal speed of sound. Since the atomic wt of water
is very close to the mean atomic wt of air this is a very small (but
easily calculated) change, unlike the change in speed due to
temperature. Since the density of atmpspheric air, an ideal gas for
all practical purposes, is proportional to 1/AbsTemp and the
small-signal speed of sound varies as sqrt[1/density], it also varies
as sqrt[AbsTemp]. So temperature compensation would be required for
an accurate measurement unless temperature is controlled.

The decrease in Q due to the increased viscosity of humid vs dry air
is very small, probably hard to detect without a lot of care.

For real entertainment, consider the non-linear finite-amplitude case
:-).
.

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